Semiconductor devices which provide power converter functionality, for example for altering DC power using a DC to DC (DC-DC) converter, are used in various capacities. For example, input DC power from one or more batteries can be converted to provide one or more power outputs at voltages which can be higher or lower than the input DC voltage. Performing a power conversion function using integrated circuits (IC's) typically requires a control circuit, a DC high-side device electrically coupled with voltage in (VIN), and a DC low-side device electrically coupled with ground. In a synchronous step-down device (i.e. a “synch buck” converter), for example, power conversion is performed to decrease voltage by alternately enabling the high-side device and the low-side device, with a switching and control function being performed by the controller circuit with high efficiency and low power loss through the device.
Power converter circuits which can operate at a high power density (for example, high voltage and high current) are needed, particularly devices which can efficiently convert high density power at a reasonable cost. One challenge with high power density is that the size of the output circuitry increases as the voltage and current rating of the converter increases. Different implementations of the controller circuit, the high-side device, and the low-side device have been used, each with its own advantages and disadvantages.
Monolithic devices could be formed which contain the controller circuit, the high-side device, and the low-side device on a single piece of silicon. In high density devices, the percent of silicon containing the controller circuitry is much smaller than the percent of silicon containing the high current output devices. The output power devices can require more than 50% of the die surface. However, forming the controller circuitry can require providing CMOS devices, bipolar devices, LDMOS devices, nonvolatile memory, diodes, resistors, capacitors, etc., and can involve the use of more than 20 mask steps during the fabrication process. In contrast, forming the output power devices can require eight or fewer mask steps for their fabrication. Because of mask misalignment and other processing issues, processing failures increase with increasing mask steps. Thus forming the controller circuitry and output devices on the same piece of silicon is inefficient and costly, because silicon formed with an eight mask process is subject to a 20 mask process failure rate and extra cost (equivalent to 12 extra mask layers). As such, monolithic devices are not used to integrate the power devices with the controller circuitry.
Co-packaged devices can include controller circuitry on one semiconductor die, the high-side device on a second die, and the low-side device on a third die. In one type of co-packaged device, the controller circuitry on one die is then connected to the high-side and low-side devices formed from standard vertical MOSFETs on the other two dies using bond wires or other connections. In another type of device, the controller circuitry on one die is the connected to the high-side device including bottom-source lateral diffusion metal oxide semiconductor (LDMOS) and a low-side vertical diffusion MOS (DMOS) device. In both of these devices, the three separate dies are then encapsulated or otherwise packaged together in one IC device. Forming controller, low-side, and high-side devices on separate dies overcomes the above-stated problems of monolithic devices. However, co-packaged devices can have problems with interconnection parasitics on the controller IC which can negatively influence device performance. This may result from parasitic inductance inherent in bond wires, electromagnetic interference (EMI), ringing, efficiency loss, etc. Higher-quality connections such as copper plate (or clip) bonding, or ribbon bonding, can be used to reduce parasitics, but this increases assembly costs. Further, co-packaging standard vertical MOSFETs can result in a circuit with parasitic inductance in series with the output node. Problems caused by parasitic inductances are well established in the art. While a capacitor can be connected to the output terminals such as the input (VIN) and ground, to compensate for the negative impact of inductances connected to these nodes, capacitances cannot be connected to internal nodes such as the Output (VOUT, also referred to as phase node or switched node).
Additionally, packages containing three separate dies have higher production costs, for example because of the large number of die attach steps (three in this example), and additional space is required for spacing between adjacent dies to allow for die attach fillets, die placement tolerance, and die rotation tolerance, which reduces the power-density which can be achieved. Examples of co-packaged devices include non-synch buck with co-packaged high-side MOSFET and external Schottky diode, non-synch buck with co-packaged high-side and low-side MOSFETs, synchronous buck with co-packaged high-side and low-side MOSFETs, boost converter with co-packaged MOSFET, and boost converter with co-packaged MOSFET and Schottky diodes.
Discrete devices can also be mounted separately to a printed circuit board. In this solution, a first packaged die containing controller circuitry is used in conjunction with a second packaged die containing a high-side MOSFET and a third package containing a low-side MOSFET. The three packages are mounted on a printed circuit board. However, this can increase packaging costs as the number of dies and separate packages which must be manufactured and handled is at least tripled, and the area used on the printed circuit board is also increased, leading to increased circuit board size.
There is a need for power converters in which device processing costs are reduced while providing a power converter device which has sufficient device electrical characteristics with low parasitic inductance and capacitance.
Co-pending U.S. patent application Ser. No. 12/470,229 titled “Co-Packaging Approach for Power Converters Based on Planar Devices, Structure and Method”, having the same inventor and assignee as the present application and incorporated herein by reference in its entirety, describes a structure for providing voltage converter power devices (high-side and low-side output devices) on a single die. A structure includes the use of a lateral diffusion MOS (LDMOS) device as a high-side device and a planar vertical diffusion MOS (VDMOS) device as the low-side device. While providing reasonable cost and manufacturability which is sufficient for many uses, a low-side planar VDMOS device may not achieve a minimum specific resistance (RDS*Area) in other uses, for example because the transistor channel is planar, the cell pitch is relatively large, and there is a parasitic junction field effect transistor (JFET) resistance between adjacent body diffusions.
It should be noted that some details of the FIGS. have been simplified and are drawn to facilitate understanding of the inventive embodiments rather than to maintain strict structural accuracy, detail, and scale.